Our goal is to define the role(s) played by the fibroblast during fibrosis, cardiac remodeling and the development of heart failure in two mouse models of human disease. Fibrosis is increasingly recognized as an important contributor to progressive heart disease and failure but there is a striking deficit in our understanding of fibroblast-based signaling and its role in these processes. We hypothesize that TGFp signaling processes that are fibroblast-based play a critical role the fibrotic response in sarcomere- based and non-sarcomere-based disease. We will precisely ablate discrete signaling pathways in the fibroblast during cardiac disease development. Two models will be used. Model 1 is a sarcomere protein- based model in which an N-terminal fragment of cardiac myosin heavy chain C (cMyBP-C) can be inducibly expressed specifically in the cardiomyocyte. This fragment uniquely presents in the failing or diseased human heart. Expression of this polypeptide results in cardiac hypertrophy, fibrosis and transition to failure. Model 2 utilizes a mutant aB crystallin (CryAB'^^^) that is causative for human skeletal and cardiac disease, but is a non-sarcomeric protein-based model of cardiac failure. This model is also characterized by hypertrophy, extensive fibrosis and heart failure. Aim 1 will test the hypothesis that canonical TGF(3 signaling plays a critical role in fibrosis during hypertrophy and failure in the cMyBP-C truncation model. The necessity and sufficiency of different pathways active in the fibroblast will be defined by breeding the transgene into a novel inducible kniockout set of mice in which we are able to ablate either canonical or non-canonical TGF(3 signaling. The mutant cMyBP-C allele will be bred into a mouse line with activated fibroblast-specific expression of inducible Cre. Offspring containing both the transgene and inducible Cre will then be bred into lines with either a smad2/3-loxP or tgfprl-loxP allele. Aim 2 will test and define the role of TGFp signaling in our CryAB^^^ model in a similar set of experiments. These aims will define the role of fibroblast-based signaling. Aim 3 willdetermine, in both the CryAB and cMyBP-C models, the importance of non-TGFp cytokine signaling during cardiac disease. The experiments outlined in Aims 1 and 2 will be repeated, but on a genetic background in which we are able to modulate non-canonical TGFp signaling in the fibroblast. We hypothesize that non-canonical TGFp signaling plays a key role in fibrosis during sarcomere-protein based cardiac disease. To'test that, we carry out the experiments described above, but in Tak1- and p38a-loxP targeted mice. Those lines will be crossed to the inducible fibroblast-specific Cre alleles. RELEVANCE (See instructions): Fibrosis often occurs during the development of heart disease. In fact, fibrosis remains a hallmark of hypertrophic cardiomyopathy and is a substrate for arrhythmogenic events, pump dysfunction and, eventually, heart failure. We propose to study how a major cell type in the heart, the fibroblast, contributes to these disease processes. Understanding the roles these cells play will open up novel therapeutic possibilities for impacting favorably on cardiac disease.